This invention relates to a lightweight, high strength and stiffness composite tennis racket assembly that provides superior performance. The invention also provides a method of making such a structure.
The earliest tennis rackets utilized frames of solid wood, but these were superseded by laminated wood construction, such as selected ash, maple and birch laminates, sometimes with built-in, steel-like fibers.
Later, tennis rackets with metal frames, such as tubular round, channel, I-beam extrusions and other configurations, were developed constructed from alloy steel, magnesium and especially aluminum.
Also, composite laminated fiber glass reinforced plastic tennis rackets have been produced by techniques such as injection molding, transfer molding and compression molding.
In copending and coassigned Application Ser. No. 720,514, filed by Andrew M. Cecka and Pol Dano on Sept. 3, 1976, there is described a tennis racket comprising an assembly of a long structural member and a short structural member, each comprising foamed plastic core integrally bonded to a shell comprising a plurality of layers of resin-coated unidirectionally oriented graphite fibers at least one of the layers having fibers oriented in a direction different from the direction of orientation in at least one other layer, the shell completely encasing the core at any transverse cross section of the structural member. Each of the structural members is prepared by arranging within a mold cavity an outer shell made of a plurality of layers of unidirectionally oriented graphite fibers and a core comprising a foamable composition, sealing the mold and activating the foamable resin composition to cause expansion and generate pressure within the mold cavity and thereby provide intimate bonding of the core to the shell.
The tennis racket of the aforementioned copending application has a handle and an ellipsoid head attached to one end thereof, with a grip element at the opposite end of the handle. One end of the long structural member is at the grip end of the handle; and the long structural member then stretches along one side of the handle, around a major portion of the head then down the other side of said handle to the grip end. The short structural member extends around a minor portion of the head, or across the throat of the racket, to complete the ellipsoid shape of the racket head. The ends of the short structural member are joined to the long structural member at junction points.
The aforementioned copending application describes the short structural element as "spliced" to the long element to obtain the assembly and complete the closed ellipsoid shape of the frame. In practice in the commercial manufacture of the tennis racket described in said application, the splicing is achieved in end extensions of the graphite fiber jackets beyond the ends of the core of the short element.
Specifically, the cross-plied graphite sheet layers wrapped around the core of the short element are wrapped to extend about 1-1/2ins. beyond the core at each end. Two 1-1/2 inch longitudinal cuts then made, 180.degree. apart, at each end, provide two tabs at each end. One tab is then overlaid (or spliced) onto the inside surface of the long structural element on the segment leading from its junction point towards the head of the racket, while its opposite tab is overlaid at the inside surface of the long structural element on the segment leading from its junction point towards the racket handle.
Tennis rackets made in the manner described above have excellent longitudinal and torsional strength characteristics. However, there have been occasional instances of cracking and failure at the junction points under high stress.